What other egg white proteins co-purify with lysozyme on CEX?

Avidin (pI 10.5) co-elutes near lysozyme. Remove by including biotin in wash buffer, adjusting NaCl gradient, or a second SEC step (68 vs 14.3 kDa).

How to Separate Lysozyme from Ovalbumin

Property Comparison

Lysozyme (Target)

Molecular Weight14,313 Da

TypeBasic Protein (Enzyme)

Charge (pH 7)+8

pI11.35

Solubility>10 mg/mL in buffer

Structure129 aa, 4 disulfide bonds

Source3.5% of egg white protein

vs

Ovalbumin (Impurity)

Molecular Weight44,287 Da

TypeAcidic Glycoprotein

Charge (pH 7)−12

pI4.5

Solubility>50 mg/mL in buffer

Structure385 aa, phosphoprotein

Source54% of egg white protein

Why This Separation Works

Lysozyme and ovalbumin have a remarkable 6.85 pH unit difference in isoelectric point—one of the largest pI gaps found in any natural protein mixture. At pH 7–8, their charges are not just different in magnitude but opposite in sign, making cation exchange chromatography extraordinarily selective:

Component	pI	Charge at pH 7.5	CEX Binding	Goes To
Lysozyme	11.35	+8 (strongly cationic)	Binds strongly	Eluate (product)
Ovalbumin	4.5	−12 (strongly anionic)	No binding (repelled)	Flow-through (waste)

Additionally, the 3× size difference (14.3 vs. 44.3 kDa) provides a secondary separation basis via ultrafiltration (30 kDa MWCO) if needed. However, the charge difference alone is sufficient for >95% purity in a single step.

Recommended Process Route

1

Egg White Dilution & Clarification

Dilute egg white 1:5 to 1:10 with 50 mM sodium phosphate buffer (pH 7.5). This reduces viscosity (egg white is ~90% water but very viscous from ovomucin). Remove insoluble ovomucin, lipids, and shell fragments by centrifugation (10,000×g, 20 min) or depth filtration.

Clarification

2

pH Adjustment to 7.0–8.0

Adjust to pH 7.0–8.0 with phosphate or Tris buffer. At this pH, lysozyme (pI 11.35) has net charge +8 and will bind cation exchange resin. Ovalbumin (pI 4.5) has net charge −12 and is electrostatically repelled. This pH maximizes the charge difference: 20 charge units apart.

Feed conditioning

3

Cation Exchange Chromatography

Load onto strong cation exchange resin (SP Sepharose Fast Flow or CM Sephadex C-50). Lysozyme binds quantitatively; ovalbumin and ovotransferrin (pI 6.0) pass through. Wash with 3–5 column volumes of equilibration buffer. Elute lysozyme with linear NaCl gradient (0–1 M) or step elution at 0.5 M NaCl. Capacity: 25–40 mg lysozyme per mL resin.

Key separation step

4

Desalting & Concentration

Remove NaCl from the eluate by diafiltration using 10 kDa MWCO ultrafiltration membrane (lysozyme at 14.3 kDa is retained). Alternatively, use gel filtration on Sephadex G-25 for buffer exchange. Concentrate to desired final concentration (typically 10–50 mg/mL) for lyophilization or formulation.

Final product

Expected Results

>90%

Lysozyme Yield

>95%

Lysozyme Purity

4 steps

Total Process Length

Research-grade lysozyme (>95% by SDS-PAGE) is achieved in a single CEX step. Pharmaceutical-grade (>98%) may require a second polishing step by size exclusion or hydrophobic interaction chromatography.

Alternative Techniques

Technique	Feasibility	Notes
Ultrafiltration (30 kDa MWCO)	Moderate	Lysozyme (14.3 kDa) permeates; ovalbumin (44.3 kDa) is retained. 3× MW ratio is borderline—some lysozyme loss and ovalbumin leakage. Better as a polishing step than primary separation.
NaCl Crystallization	Good	Classic method: add 5% NaCl to egg white at pH 9.5. Lysozyme crystallizes selectively due to its high pI. Simple but lower purity (~80%) than chromatography. Used historically at industrial scale.
Affinity Chromatography (chitin)	Good	Lysozyme is a muramidase that binds chitin (GlcNAc polymers) with high affinity. Chitin beads give excellent selectivity. Higher cost than CEX; mainly used for research-scale.
Anion Exchange (AEX)	Moderate	Reverse strategy: ovalbumin (−12) binds AEX; lysozyme (+8) passes through. Works but lysozyme co-elutes with other basic egg white proteins (ovomucoid, avidin).

Frequently Asked Questions

Why is lysozyme/ovalbumin considered a “textbook” separation?

The 6.85 pH unit pI difference (11.35 vs. 4.5) is among the largest found in any natural protein pair. At physiological pH, the proteins carry opposite charges (+8 vs. −12), making ion exchange separation nearly trivial. This system is used in virtually every biochemistry teaching lab and chromatography textbook as the canonical example of protein ion exchange.

Can I separate lysozyme from ovalbumin by size alone?

Technically possible but challenging. The MW ratio is 3.1× (44.3/14.3 kDa), which is at the lower limit for effective UF separation. A 30 kDa MWCO membrane would partially separate them, but with significant lysozyme loss (30–40%) in the retentate and some ovalbumin leakage into the permeate. CEX is far more selective and gives better yield.

What other proteins in egg white co-purify with lysozyme on CEX?

Avidin (pI 10.5, 68 kDa) and ovomucoid (pI 4.1, but glycosylated forms can bind weakly) are the main contaminants. Avidin binds CEX at pH 7–8 and co-elutes near lysozyme. It can be removed by: (1) including biotin in the wash buffer (avidin binds biotin with extreme affinity), (2) adjusting NaCl gradient to resolve the two peaks, or (3) a second SEC step (68 kDa vs. 14.3 kDa is easily resolved).

What is the industrial demand for egg white lysozyme?

Lysozyme is used as a natural antimicrobial preservative in cheese (prevents late blowing by Clostridium tyrobutyricum), wine (malolactic fermentation control), and as a pharmaceutical excipient. Global production is ~100 tonnes/year, primarily from hen egg white by ion exchange. Price ranges from $50–200/kg depending on purity grade.

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